1. Field of the Invention
This invention relates to a test circuit for indicator lamps and more particularly to an improved circuit for simultaneously testing indicator lamps including builtin protection against current feedback upon component failure.
2. Description of the Prior Art
On modern machinery there are many applications where multiple indicator lights are used. It is advantageous to be able to rapidly test many indicator lamps by depressing a single pushbutton. A problem with some of the prior art test circuits is that many relays or multi-pole pushbuttons are required in the test circuit. In some of the prior art indicator circuits, the normal signal circuits, the test circuit, and the indicator lamp circuits are all operated at the same potential. It is desirable to have a circuit in which the test pushbutton and the indicator signal are at a higher normal operating voltage such as 120 volts AC while the indicator lamp is maintained at a low voltage such as 6 volts for system safety and cost.
In prior art U.S. Pat. No. 3,040,243 issued June 19, 1962 to I. F. Weiss, a test circuit for an indicator system utilizing a single test pushbutton is disclosed. A problem with this circuit as disclosed in Weiss is that a separate transformer is required for the test circuit. The separate test transformer can cause polarity and installation difficulties. In this prior art circuit the diodes are disposed on the low voltage side of the transformers. In a low voltage indicator circuit the voltage drop across the diodes can be significant.
It is also desirable to have a high reliable fail safe test circuit. The test circuit disclosed in U.S. Patent application Ser. No. 398,353 now U.S. Pat. No. 3,840,801 overcomes many of the problems associated with prior art test circuits. Normally, an indicating light is wired in parallel with a relay coil to indicate an energized condition. For bulb test, rectified AC is fed into the primary of the transformer which is normally blocked by a rectifier on the line between the transformer and relay coil. If one or both rectifiers should short and let current pass in the reverse direction, it would be possible that the relay coil would become energized. The circuit as disclosed in copending U.S. application Ser. No. 398,353 now U.S. Pat. No. 3,840,801 can possibly have an isolation problem between the tests circuit and the signal circuit upon component failure. It is desirable to have a signal and test circuit combination which is highly reliable and not susceptible to showing erroneous indications. It is desirable that a malfunction of one component of the indicator lamp circuit will not cause erroneous signal lamp lighting or remote device activation. It is also desirable that isolation between the test circuit and the signal circuits be maintained upon failure of some of the isolating components.